chore(gatsby-plugin-mdx): Upgrade MDX and snapshots (#26143)

Starting in version 1.6.16, the makeShortcode function
is only added to compiled MDX when it is needed. This
caused the gatsby-plugin-mdx snapshots to fall out of
date. MDX is no bumped to 1.6.16 with the snapshots
updated.

Related:

- gatsbyjs/gatsby#24595 (comment)
- mdx-js/mdx#1088

yarn.lock

@@ -1607,7 +1607,12 @@
   dependencies:
     "@types/node" "*"
 
-"@types/node@*", "@types/node@^8.5.7":
+"@types/node@*":
+  version "14.0.14"
+  resolved "https://registry.yarnpkg.com/@types/node/-/node-14.0.14.tgz#24a0b5959f16ac141aeb0c5b3cd7a15b7c64cbce"
+  integrity sha512-syUgf67ZQpaJj01/tRTknkMNoBBLWJOBODF0Zm4NrXmiSuxjymFrxnTu1QVYRubhVkRcZLYZG8STTwJRdVm/WQ==
+
+"@types/node@^8.5.7":
   version "8.10.61"
   resolved "https://registry.yarnpkg.com/@types/node/-/node-8.10.61.tgz#d299136ce54bcaf1abaa4a487f9e4bedf6b0d393"
   integrity sha512-l+zSbvT8TPRaCxL1l9cwHCb0tSqGAGcjPJFItGGYat5oCTiq1uQQKYg5m7AF1mgnEBzFXGLJ2LRmNjtreRX76Q==